Understanding GIS Data¶
Question:
How could we tell someone on the other side of the world where we want to meet them for coffee?
It depends on how we talk about the world.
- Do we use the same units?
- Do we understand direction the same?
Thankfully, we can communicate mathematically.
So About the Earth¶
Well it's not flat.
Earth's Geometry¶
The earth actually isn't even a perfect sphere,
especially given the topographic changes of mountain ranges and valleys.
Sphereoid¶
We call the shape of the Earth a spheroid.
A sphereoid is defined by:
- Semi-Major Axis
- Semi-Minor Axis
Spheroids¶
And there are many approximations (meters):
- Sphere; 6370997; 6370997
- Clarke (1866); 6378206.4; 6356583.8
- GRS80 (1980); 6378137; 6356768.33
- WGS1984 (1984); 6378137; 6356752.31
Datum¶
A datum is built on top of the selected spheroid and can incorporate local variations in elevation.
Basically, we are setting the origin of our sphere as if it were placed on the earths actual geoid.
For example: Bellingham, WA
| Datum | Longitude | Latitude |
|---|---|---|
| NAD 1927 | -122.46690368652 | 48.7440490722656 |
| NAD 1983 | -122.46818353793 | 48.7438798543649 |
| WGS 1984 | -122.46818353793 | 48.7438798534299 |
- NAD27 is quite different becasue it uses different spheroid and daturm
- NAD83 and WGS84 express coordinates that are nearly identical
- same spheroid
- different dataum
Break down a sphere into:
- Latitute; parallels
- Longitude; meridians
Origins on the Earth:
- Prime Meridian
- Equator
Together form: Graticule
Measurements on a Sphere¶
Both of these are measurements from the Earth's center to a point of the Earth's surface.
These two allow us to find any place on Earth.
Cool!!! And it's called:
Geoographic Coordinate System¶
What is this actually measuring...?
- Degrees
Units:
- Decimal Degrees
- Sexagesimal Degree: Degrees, Minutes, Seconds
Well what does that mean...?
Described as a Coordinate Pair: (x,y) <- (in most cases)
University of Puget Sound:¶
Decimal Degrees:
- Latiutude: 47.261780
- Longitude: -122.481689
- (47.261780, -122.481689)
Sexagesimal
- 47 Degrees 15 Minutes 42.4080 Seconds
- 122 Degrees 28 Minutes 54.0804 Seconds
DD -> DM:
absDegrees = |decimal degrees|
floorAbsDegrees = |absDegrees|
degrees = sgn(decimal degrees) floorAbsDegrees
minutes = |60 (absDegrees - floorAbsDegrees)|
seconds = 3600 (absDegrees - floowAbsDegrees) - 60 minutes|
Measurements on a Sphere Continued¶
1 degree of longitude at the equator: 111.321 KM
1 degree of longitude at 60 degrees of latitue: 55.802 KM
Woah, thats weird...
Ends up, most people are not good at thinking in 3 dimensional space...¶
University of Puget Sound:¶
UTM Zone 10 (FT)
- Easting: 539230.069260529
- Northing: 5234383.60405841
- (539230.069260529, 5234383.60405841)
WA State Plane South (FT):
- Latiutude: 1148411.465
- Longitude: 709443.892
- (1148411.465, 709443.892)
This is Called a Projection¶
We can use different types of math here to be able to flatten the sphere to a 2d plane.
Different projection formulas result in different types of projections.
Additionally, different projection formulas can be used to preserve angles, area, or distance.
Equal-Area Projections¶
- Preserves area
Shape, angle, scale may all be distorted. Graticules may not intersect at right angles.
Eqidistant Projections¶
- Preserves distance
Scale is only preserved in some areas of a map.
True-Direction Projections¶
- Shows the shortest route
Scale is only preserved in some areas of a map.
Different Types of Projections¶
Albers Equal Area¶
Two standard parallels
Commonly used for maps of the conterminous United States.
- Conic
- Equal-area
- Shapes, directions, angles, and distances are generally distorted.
- The scale, directions, and distances are true only along the standard parallels
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Lambert Conformal Conic¶
Directions, angles, and shapes are maintained at infinitesimal scale.
<cartopy.mpl.gridliner.Gridliner at 0x126f25ee0>
Equirectangular¶
The meridians and parallels are equally spaced straight lines forming a Cartesian grid.
Each rectangular grid cell has the same size, shape, and area only in the projected space.
- North, south, east, and west directions are always accurate.
- General directions are distorted, except locally along the standard parallels.
<cartopy.mpl.gridliner.Gridliner at 0x126f9fa30>
Orthographic¶
- Neither conformal nor equal-area
- Shapes, areas, distances, directions, and angles are all generally distorted.
- Only the center of the map is free of distortion.
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Transverse Mercator¶
Used for mapping large-scale or smaller areas with predominantly north-south trending extents.
Conformal map projection
- Preseves Distance and Angles
- Can use the Pythagorean theorem to be able to calculate distances
- Great for in the field
<cartopy.mpl.feature_artist.FeatureArtist at 0x127b33100>
Universal Transverse Mercator¶
The UTM system divides the Earth into 60 zones, each 6° of longitude in width.
- Conformal Projection
UTM Zone 10 - Washington State¶
| UTM Zone | Central Meridian | UTM Zone Extent |
|---|---|---|
| 10 | 123°W | 126°W-120°W |
- Preserves distance in North-South Direction
State Plane¶
State Plane system of large-scale conformal map projections originally created in the 1930s to support surveying, engineering, and mapping activities throughout the U.S. and it's territories.
Loosely based on:
- Transverse Mercator projection
- Lambert conformal conic projection
Problems - Puget Sound Metro
King County: "Washington State Plane North"
Pierce County: "Washington State Plane South"
I have even encountered a custom projection to deal with this issue!!
Takeaways¶
There are a variety of different projections for different reasons and I don't expect you to know them all.
However, you will need to know how to find a projection of a data source, map, and other services to be able to work with data.
Each agency typically has their own standards and you will be asked to use that when working with that data.
Resources¶
Reading¶
Learning ArcGIS Pro 2: Chapter 3